Related art is known in which a vehicle speed is obtained based on detection signals from wheel speed sensors and then displayed on a speedometer (refer to Japanese Laid-open Patent Publication No. HEI 8-268252). In the case where front and rear wheels have different diameters, the vehicle speed is calculated based on detection signals from either the front wheel or the rear wheel. For example, in a case of a rear wheel drive vehicle, the rear wheels are the object of vehicle speed calculation and the vehicle speed is obtained based on the detection signals from wheel speed sensors provided on rear wheels.
In a case where the front and rear wheels have different diameters and where the detection signals from the wheel speed sensor on the rear wheels that are used in the vehicle speed calculation are thought to be unreliable, for example, when a normal tire has been replaced with a spare, a correct vehicle speed cannot be obtained from the detection signals from the wheel speed sensor. Thus the correct vehicle speed cannot be obtained from the wheel speed sensor. In this case, the vehicle speed obtained from the vehicle speed sensor on the front wheels is corrected using a tire diameter ratio of the front and rear wheels, whereby the speedometer can display the actual vehicle speed after diameter correction.
Another related art obtains a vehicle speed from a mean value of a plurality of vehicle wheel speeds, based on detection signals from a plurality of wheel speed sensors. This mean value of the vehicle wheel speeds may be obtained, for example, based on detection signals from a plurality of wheel speed sensors provided on respective driving wheels. In a case where the vehicle speed is obtained in this manner, if it is probable that a detection signal from a sensor is not reliable due to noise of the detection signal from the wheel speed sensor, the vehicle speed is obtained solely from vehicle wheel speeds from other sensors.
In such art in which the vehicle speed based is obtained on the detection signals from the plurality of sensors, when reception of a detection signal from a wheel speed sensor is disabled by wire breakage, the vehicle speed is obtained solely from detection signals from other wheel speed sensors with no wire breakage.
In the case of diameter correction as described above, the vehicle speed is calculated by the rear wheels as the objects of calculation before the diameter correction. On the other hand after the diameter correction, the vehicle speed is calculated with the front wheels as the objects of calculation. Accordingly, the vehicle speed is calculated while taking into consideration the difference between the diameters of the front and rear wheels. This will cause a change in the vehicle speed before and after the diameter correction and, in a case where the speedometer display changes abruptly, the driver will be caused to feel discomfort.
In order to avoid the wheel speed sensor detection signal affected by the noise, or the wheel speed sensor detection signal from the wire-broken sensor, being used for vehicle speed calculation as described above, a previous vehicle speed may be used. However, in this case, an abrupt change in the speedometer display may cause the driver to feel discomfort if the newly obtained vehicle speed is different from the previous vehicle speed.